Fluid filter systems utilizing central filter cartridges are well known. For example, U.S. Pat. No. 4,561,979 to Harms et al., which is incorporated by reference into this disclosure, discusses a fluid filter system having a central filter cartridge. In the Harms et al. system, a central filter cartridge defines a filtering partition between an outer, unfiltered fluid chamber and an inner, filtered fluid chamber. Unfiltered fluid is introduced to the outer chamber through an inlet in the outer housing of the system. Due to a pressure differential, the fluid is forced through a pleated filter media sleeve of the filter cartridge, which is disposed in a central portion of the outer housing. The filtered fluid exiting the filter media travels upwardly in an annular volume defined by the filter sleeve and a smaller diameter, outlet tube disposed in a central cavity of the filter cartridge. The filtered fluid, with associated gases, is forced upwardly into an upper chamber where the flow is reversed and evacuated downwardly through the outlet tube.
In the Harms et al. filter, unfiltered fluid enters the inlet chamber through an inlet port in the bottom surface of the filter housing. The upward, axial flow relative to the pleated filter media can impart forces on the individual filter media pleats, causing the pleats to compress against one another or "blind off." Blinding off reduces the filter media surface area available for filtering and can affect the flow efficiency of the system. Additionally, the reduced surface area can cause concentrations of filtered debris on the available media surface area, thereby requiring premature replacement or servicing of the filter cartridge.
An improved cartridge filter system that reduces the effects of this blinding off phenomenon is described in co-pending U.S. patent application Ser. No. 543,326 to Harms et al., which is incorporated by reference into this disclosure. This filter system is constructed to create rotational flow around the pleated filter media to reduce the likelihood of blinding off.
In this rotational flow filter system, unfiltered fluid is introduced into the filter system through tangential inlet passages in the side wall of the filter housing. The entering fluid flows tangentially to the housing contour and is induced into rotational flow by the curvature of the housing. The rotational flow imparts substantially uniform forces tangential to the pleats of the filter media. Thus, the pleats are urged in the same direction in a substantially uniform manner, thereby reducing the likelihood of blinding off.